Studies of the interaction of hematopoietic cells and viruses have mainly concentrated on members of the Parvoviridae and with our interest in hepatitis-associated aplastic anemia, novel putative hepatitis agents. B19 parvovirus infects erythroid progenitor cells and infection in humans causes both the hematologic syndromes transient aplastic crisis and pure red cell aplasia as well as the common childhood exanthem fifth disease. Recently, studies from France have described a variant of B19, V9, which is not detected by PCR assays designed for B19 detection. We obtained a clone of this variant, and have developed assays that will detect both B19 and related variants. Retrospective and prospective testing of samples sent to the laboratory for B19 detection did not detect V9, but we have identified a second variant, in one sample sent from Italy in 1992. This variant has been cloned and sequenced, and studies are in progress to look for other variants. In addition we have developed new assays to detect B19 viral RNA transcripts and infectious virus from patient samples, and are using these assays to look for evidnece of B19 persistence in a variety of tissues, including liver. Our studies suggest that B19 DNA may persist at low levels in tissues without medical consequence. Other parvoviral studies include development of adeno-associated virus vectors based on other adeno-associated virus strains, and comparative studies of hemagglutination and tissue tropism of these viruses. In addition, we have pursued further studies to determine the early events of AAV infection, including the nature of the cell surface receptor for AAV-2. Our studies to identify the etiological agent of hepatitis-associated aplastic anemia continue with the collection of epidemiological data and animal studies. Currently we are looking at the immune profile of livers from patients with hepatitis-aplasia and controls. In addition, we have evaluated the role of the putative novel hepatitis virus, TTV. This virus was first identified in a patient with transfusion-associated hepatitis, but its role in human disease still remains unclear. We have developed PCR primers to detect TTV, and with these assays we can detect TTV sequences in ~ 80% of American blood donors, but the high degree of sequence variation suggests that current assays may not detect all TTV strains. In addition we have expressed part of the TTV large ORF to develop serological agents for TTV detection. Studies of the interaction of different TTV strains, with hematopoietic cells are continuing.